NASA image acquired November 11-12, 2012.

On November 12, 2012, the Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP satellite captured the top nighttime image of city, village, and highway lights near Delhi, India. For comparison, the lower image shows the same area one night earlier, as observed by the Operational Line Scan (OLS) system on a Defense Meteorological Satellite Program (DMSP) spacecraft.

Since the 1960s, the U.S. Air Force has operated DMSP in order to observe clouds and other weather variables in key wavelengths of infrared and visible light. Since 1972, the DMSP satellites have included the Operational Linescan System (OLS), which gives weather forecasters some ability to see in the dark. It has been a highly successful sensor, but it is dependent on older technology with lower resolution than most scientists would like. And for many years, DMSP data were classified.

Through improved optics and “smart” sensing technology, the VIIRS “day-night band,” is ten to fifteen times better than the OLS system at resolving the relatively dim lights of human settlements and reflected moonlight. Each VIIRS pixel shows roughly 740 meters (0.46 miles) across, compared to the 3-kilometer footprint (1.86 miles) of DMSP. Beyond the resolution, the new sensor can detect dimmer light sources. And since the VIIRS measurements are fully calibrated (unlike DMSP), scientists now have the precision required to make quantitative measurements of clouds and other features.

“In contrast to the Operational Line Scan system, the imagery from the new day-night band is almost like a nearsighted person putting on glasses for the first time and looking at the Earth anew,” says Steve Miller, an atmospheric scientist at Colorado State University. “VIIRS has allowed us to bring this coarse, blurry view of night lights into clearer focus. Now we can see things in such great detail and at such high precision that we’re really talking about a new kind of measurement.”

Unlike a film camera that captures a photograph in one exposure, VIIRS produces an image by repeatedly scanning a scene and resolving it as millions of individual picture elements, or pixels. The day-night band goes a step further, determining on-the-fly whether to use its low, medium, or high-gain mode. If a pixel is very bright, a low-gain mode on the sensor prevents the pixel from over-saturating. If the pixel is dark, the signal will be amplified.

“On a hand-held camera, there’s a nighttime setting where the shutter will stay open much longer than it would under daylight imaging conditions,” says Chris Elvidge, who leads the Earth Observation Group at NOAA’s National Geophysical Data Center. “The day-night band is similar. It increases the exposure time—the amount of time that it’s collecting photons for pixels.”

NASA Earth Observatory image by Jesse Allen and Robert Simmon, using Suomi NPP VIIRS and DMSP OLS data provided courtesy of Chris Elvidge (NOAA National Geophysical Data Center). Suomi NPP is the result of a partnership between NASA, NOAA, and the Department of Defense. Caption by Mike Carlowicz.

Instrument: Suomi NPP - VIIRS

Credit: NASA Earth Observatory

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